One type of semiconductor pressure sensor utilizes a two-wafer construction formed of a flexible silicon diaphragm laminated to a non-deflecting substrate, as disclosed in Knecht et al., U.S. Pat. No. 4,790,192. A sensing element for measuring diaphragm deflection, typically by capacitive or resistive techniques, is secured to the diaphragm. The deflection of the diaphragm, due to pressure variations applied to the diaphragm, causes changes in the electrical characteristics of the sensing element. These changes can be measured and converted into a pressure-responsive electrical signal for further processing.
Pressure sensors can be constructed to measure absolute pressure and to measure differential pressure. In one form of an absolute pressure sensor, a flexible diaphragm spans a sealed cavity in the substrate. As a first fluid pressure external to the sensor varies, the diaphragm deflects and activates the sensing element. In a corresponding form of a differential pressure sensor, a first fluid pressure is applied to the top face of a diaphragm that spans a cavity in the substrate, and the bottom face of the diaphragm is exposed to a second fluid pressure. The bottom face, or underside, of the diaphragm is typically exposed by forming a port through the substrate. The flexible diaphragm in the differential pressure sensor deflects in response to the difference between the external pressures applied to the top of the pressure sensor and to the bottom of the pressure sensor. Accordingly, differential pressure sensors generally are suited to situations where two distinct fluid pressures can be applied to opposing sides of the sensor diaphragm.
Processes used to manufacture differential pressure sensors having semiconductor structures include numerous difficult manufacturing steps. In particular, etching techniques used to form the port through the substrate to the bottom face of the diaphragm are time consuming and expensive. According to one practice of this process: a cavity is formed in the top surface layer of a substrate, an oxide spacer is deposited in the cavity, and a semiconductor layer is applied over the oxide spacer. Thereafter, the oxide spacer is etched from the reverse or backside of the substrate to define a diaphragm that is free-standing over the cavity, i.e., the diaphragm spans across the cavity. This back-side etching process requires numerous processing steps and is relatively complex and costly. Furthermore, difficulties in maintaining the processing steps within the required tolerances create difficulties in forming high yields of semiconductor pressure sensors over time.
Therefore, it is an object of this invention to provide a differential pressure sensor which can be manufactured in a cost effective manner with a relatively high yield of acceptable sensors.
An additional object of the invention is to produce a compact pressure sensor that uses only a single semiconductor layer and that does not require alignment between a plurality of layers.
It is a further object of the invention to provide a pressure sensor capable of measuring the differential between two distinct fluid pressures applied to the same side of the pressure sensor.
Other general and specific objects of this invention will be apparent and evident from the accompanying drawings and the following description.